RFR: 8325169: Reduce String::indexOf overheads

[Raffaello Giulietti]

On Fri, 2 Feb 2024 16:20:21 GMT, ExE Boss <duke at openjdk.org> wrote:

>> This patch streamlines and specializes various `String::indexOf` methods. Mainly avoids the need for clamping and doing checks that are redundant in almost all cases, moving the checks to the API boundary where they are needed. 
>> 
>> This improves performance both at peak and during startup/warmup. Since indexOf is heavily used in bootstrapping code it makes sense to slightly dial back abstraction and delegation, which in this case also brought some benefit to peak performance.
>> 
>> Testing: tier1-3
>
> src/java.base/share/classes/java/lang/String.java line 2506:
> 
>> 2504:         fromIndex = Math.max(0, fromIndex);
>> 2505:         return isLatin1() ? StringLatin1.indexOf(value, ch, fromIndex, value.length)
>> 2506:                 : StringUTF16.indexOf(value, ch, fromIndex, value.length >> 1);
> 
> This needs to include the check for `fromIndex >= this.length()`:
> Suggestion:
> 
>         fromIndex = Math.max(0, fromIndex);
>         int toIndex = length();
>         if (fromIndex >= toIndex) {
>             return -1;
>         }
>         return isLatin1()
>                 ? StringLatin1.indexOf(value, ch, fromIndex, toIndex)
>                 : StringUTF16.indexOf(value, ch, fromIndex, toIndex);

I don't think so.
If you deeply follow the invoked `indexOf()` methods, there's either a check later, or the loop conditions are false on entry (although I'm not sure about the intrinsic methods).

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PR Review Comment: https://git.openjdk.org/jdk/pull/17685#discussion_r1476293651